In recent years, in internal combustion engines for vehicles, the development and practical use of hybrid vehicles having an internal combustion engine and a power source (e.g., a generator motor, etc.) other than the internal combustion engine are being in progress for the purposes of environmental protection and improvement in fuel economy.
In such a hybrid vehicle, either one of the internal combustion engine and the power source other than the internal combustion engine is selectively used in accordance with the traveling condition of the vehicle.
For example, in order to suppress unnecessary fuel consumption during idling, there has been proposed a technique called idle stop.
The idle stop is a technique that stops an internal combustion engine when a vehicle is stopped by a traffic signal at an intersection or the like, and thereafter restarts the internal combustion engine by means of a generator motor when the intention of a driver to start the vehicle (an operation of an accelerator, a brake or the like) is sensed.
As a generator motor installed on such a kind of hybrid vehicle, there is employed a three-phase, alternating current synchronous generator motor, and when the generator motor is used as an electric motor, electric power is supplied to the generator motor through a power converter that converts DC power from a storage battery (e.g., a battery) into AC power.
However, when an induced voltage generated by the generator motor becomes higher than a battery voltage during the generator motor operates as an electric motor, it becomes unable to supply electric power to the generator motor. Here, note that the induced voltage from the generator motor is proportional to the rotational speed of the generator motor, the number of turns of an armature winding and a main magnetic flux.
In addition, at this time, in order to start the internal combustion engine, the generator motor is required to have a characteristic that can raise its rotational speed to the vicinity of the idle rotational speed of the engine, so it is necessary to make the generator motor operate as an electric motor up to a rotational speed equal to or higher than the idle rotational speed.
Accordingly, some contrivance is required to prevent the induced voltage from reaching the battery voltage even if the rotational speed of the generator motor rises to equal to or higher than the idle rotational speed, so for example, in the design of the generator motor, a technique such as reducing the number of turns of the armature winding to a required minimum is adopted.
On the other hand, when the generator motor is used as a generator, the current to be supplied to a field winding is adjusted by making use of the induced voltage, similar to a vehicular generator, so that desired AC power is generated from the armature winding, and this generated power is rectified (DC converted) by a diode and is supplied to the battery.
Of course, the generator motor is required to operate as a generator after the starting of the internal combustion engine, so the generator motor need be designed to execute a power generation operation even at the idle rotational speed.
However, as stated above, in the design of the generator motor, in case where the number of turns of the armature winding is reduced from that for an ordinary vehicular generator so as to suppress the rise of the induced voltage, there is a possibility that the power generation characteristic required in a low-speed rotational region can not be satisfied.
Accordingly, there has been proposed a technique in which in a low-speed rotational region, power generating operation is carried out in a first mode by a compensation current for phase control of an armature winding and a current to be supplied to a field winding, and thereafter, when the rotational speed is increased to raise an induced voltage to a satisfactory extent, power generating operation is performed in a second mode by a field duty that controls the current to be supplied to the field winding (see, for example, a first patent document).
However, in load response control of the generator motor having a plurality of power generating operation modes, switching between a first mode in which the rotational speed is low and a second mode in which the rotational speed is high need be seamlessly effected or succeeded from one to another, but in the above-mentioned first patent document, no consideration is given to this matter.
In addition, control should be performed in consideration of a variety of electric load conditions of a vehicle and a rotational speed condition of an internal combustion engine (engine), but in the above-mentioned first patent document, no consideration is given to this matter, either.
[First Patent Document] Japanese patent No. 3517405